Field of the Invention
The invention relates to methods for removing oxidic noxious substances in an oxygen-containing exhaust gas, in particular to clean the exhaust gas of an internal combustion engine or of another machine which is operated with fossil fuel, such as a diesel engine. The invention also relates to a device for carrying out the methods and to an engine that is operated with the specified methods and the associated device.
Exhaust gases from combustion processes constitute one of the main sources of atmospheric pollutants. These include, in particular, nitrogen oxides, sulfur dioxides, hydrocarbons, carbon monoxide, soot or the like. The three-way catalytic converter, which effectively eliminates nitrogen oxides, hydrocarbons and carbon monoxide, is the state of the art specifically for a spark-ignition engine which operates with an excess air factor .LAMBDA.=1. The catalytic converter is suitable for a diesel engine or a lean-mix spark-ignition engine (.LAMBDA.&gt;1) to only a limited degree because only the hydrocarbons and carbon monoxide are largely decomposed, but the nitrogen oxides cannot be reduced owing to the high oxygen content in the exhaust gas. Until now there has not been any type of catalytic converter with a sufficiently high noxious-substance decomposition rate and service life for these types of engines.
Various approaches to eliminating the noxious substances in the exhaust gases of diesel engines or lean-mix.spark-ignition engines are known from the prior art. In this respect, the so-called selective catalytic reduction (SCR=selective catalytic reduction) of nitrogen oxides is becoming increasingly significant.
For direct-injecting (DI) spark-ignition engines which are operated with a lean mix under partial load, i.e. with an air excess factor (.LAMBDA.&gt;1), but with .LAMBDA.=1 under full load, hydrocarbons (HC=hydrocarbons) are the reduction substances in discussion, which, however, have to be added in excess according to the prior art in order to achieve effective reduction (see for example A. Fritz, V. Pitchon: "The Current State of Research on Automotive Lean NOx Catalysis", Applied Catalysis B: Environmental, Vol. 13, pages 1-25 (1997)). The reason for this is that the hydrocarbons are also oxidized catalytically by the oxygen contained in the exhaust gas. This process becomes more probable as the temperature rises and thus sets an upper temperature limit of, for example, 550.degree. C. for the use of the HC-SCR catalytic converters. The lower limit is given by the use of No reduction and, depending on the material of the catalytic converter, is 300.degree. C. and above. A further concept for DI spark-ignition engines is associated with the term "NOx Storage Catalytic Converters" (see for example Automotive Engineering, Vol. 105, issue 2, pages 133-135 (1997)). Here, the nitrogen oxides are stored during lean operation and catalytically reduced in cyclically occurring short phases with extremely rich operation, and thus with a high emission of hydrocarbons. The hydrocarbons are not used efficiently according to this prior art either.
In diesel vehicles, the principle involved makes rich operation impossible from the outset. For this reason, the reduction substance used here is ammonia that is made available to the SCR process by a urea hydrolysis (ammonia SCR and urea SCR). This avoids a situation in which, in particular for mobile use, it is necessary for the user to carry ammonia directly with him. In practice, in mobile use, reduction rates of nitrogen oxides of more than 70% are achieved. A peculiarity of this method is that urea has to be carried along in a supplementary tank in the vehicle. This method operates at temperature intervals of approximately 200.degree. C. to a maximum of 550.degree. C.
Generally, it can therefore be claimed that SCR methods experience problems in the warm-up phase of engines and during low-load operation, for example in the town cycle, but also under full-load conditions with exhaust-gas temperatures of over 600.degree. C. in some cases.
By use of plasma-chemical methods and associated devices, as are described in Published, Non-Prosecuted German Patent Applications DE 42 31 581 and DE 44 16 676 A, International Patent Disclosure WO 97/03746 A and U.S. Pat. No. 5,476,539, it is also possible to reduce the nitrogen oxides from engine exhaust gases. Here, so-called non-thermal normal-pressure gas discharges are used to initiate chemical reactions in a plasma which can lead to decomposition of the nitrogen oxides.
"Non-thermal normal-pressure gas discharges" are generally understood as gas discharges that burn under normal pressure and in which high-energy electrons initiate plasma processes without the gas being significantly heated in the process. These include dielectrically impeded ("silent") discharges, corona discharges and pulsed glow discharges (see for example B. Eliasson, U. Kogelschatz: "Nonequilibrium Volume Plasma Chemical Processing", IEEE Trans. Plasma Sci. Vol. 19, No. 6, pages 1063-1077 (1991)). A peculiarity of these plasma-chemical methods is that they oxidize the nitrogen oxides mainly to form NO.sub.2 and HNO.sub.3 and only reduce them to the desired products N.sub.2 and O.sub.2 to a small degree and with comparatively high expenditure of energy. The combination, proposed in U.S. Pat. No. 3,983,021 and in Published, European Patent Application EP 0 659 465 A2, of a gas discharge and a catalytic converter in direct contact has also previously not been successful. This is due, inter alia, to the fact that a series of catalytic converters cannot be used as the dielectric because in the presence of certain exhaust-gas components, for example heavy hydrocarbons, they form a weak electrical conductivity which, in particular, impedes or entirely prevents the operation of the silent discharges. Furthermore, the document does not provide for the addition of a reduction substance or even excludes it as unsuitable for cleaning the exhaust gases of motor vehicles. A concrete approach for effective cleaning of exhaust gas specifically at low exhaust-gas temperatures is not given in the publication.
In addition, Published, Non-Prosecuted German Patent Application DE 195 10 804 A describes a method for reducing the nitrogen oxide in exhaust gases of internal combustion engines, in which the exhaust gases having excess oxygen are placed in contact with a catalyst while a selectively acting gaseous reduction substance is added. In the associated device, before entering into contact with the catalyst the reduction substance is converted into the high-pressure plasma state with an overwhelming formation of radicals. In contrast, in Published, European Patent Application EP 0 585 047 A, in order to clean the exhaust gases of internal combustion engines, the exhaust gas firstly passes through an electrical low-temperature discharge before it arrives at a converter with a catalyst for converting the noxious substances. Reduction is not mentioned in this reference.
Furthermore, in Published, Non-Prosecuted German Patent Application DE 21 26 228 A a method and a device for cleaning exhaust gas in motor vehicles is described, in which the exhaust gas is firstly fed to an electrostatic filter before it is conducted through a catalytic exhaust-gas reactor. The filter is intended to charge particles floating in the spatial discharge field of a corona discharge. A gas discharge is not realized here. Finally, in order to remove NOx from an exhaust gas, Published Japanese Abstract JP 63-242323 A proposes treatment using a discharge plasma before the exhaust gas is conducted with reducing effect through a catalyst bed based on platinum rhodium (PtRh).